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MODULATING THE TEMPERATURE COEFFICIENT OF THE PERMITTIVITY BY SUBSTRATE INDUCED TEXTURIZATION IN BaO-Ln2O3-TiO2 THICK FILMS
Keywords: Thick films, Texturization, Permittivity
BaO-Ln2O3-TiO2 (Ln=La, Nd, Sm) compositions present high quality factor, Q, high permittivity and low temperature coefficient of capacitance (or of the permittivity), TCC (or TCɛr), which make them potential candidates for resonators in telecommunications devices, operating at microwave frequencies. Near-zero values of TCɛr are very important to achieve signal stability whenever the components are subjected to thermal fluctuations. As an alternative to the common composition manipulation to tune the TCɛr, the constraint from the substrate during the sintering of thick films is here successfully used for the same purpose. In thick films processing, powder layers are constrained by an underlying substrate during sintering, inhibiting in-plane shrinkage. This may hinder densification as well as lead to microstructure anisotropy development. Particularly in materials with anisotropic crystallographic structures, this effect may be maximized and intentionally used for microstructure and properties design, as is here shown for BaLa4Ti4O15 (BLT) and Ba4.5Nd9Ti18O54 (BNT). BLT and BNT powders were prepared by the conventional mixed oxide method and deposited on platinum-foil substrates by electrophoretic deposition. During the constrained sintering highly dense and texturized films were obtained. Detailed microstructural and structural analysis showed that platelet particles in the case of BLT and needle-like grains in BNT, largely anisometric, have grown with the c-axis preferentially aligned parallel to the substrate. It is shown that the constraint from the substrate is responsible for the enhancement of the anisotropic grain growth. This texturized grain growth has an important effect on the dielectric properties: with the increase of the grain orientation, the relative permittivity decreased and TCɛr changed monotonically from negative to positive, in both cases, without significant effect on the quality factor. In consequence, TCɛr can be easily modulated to ~0 by controlling the texturization degree induced by the substrate with the properly handling of sintering variables, namely the sintering temperature and time, as well as an appropriate substrate choice.
Ana Senos, Professor
University of Aveiro, CICECO
Aveiro, Aveiro
Portugal


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